Xen: properly bound buffer access when parsing cpu/*/availability

At the same time reduce the local buffers to 16 bytes each.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/cpu_hotplug.c
+++ b/drivers/xen/cpu_hotplug.c
@@ -25,10 +25,10 @@ static void disable_hotplug_cpu(int cpu)
 static int vcpu_online(unsigned int cpu)
 {
 	int err;
-	char dir[32], state[32];
+	char dir[16], state[16];
 
 	sprintf(dir, "cpu/%u", cpu);
-	err = xenbus_scanf(XBT_NIL, dir, "availability", "%s", state);
+	err = xenbus_scanf(XBT_NIL, dir, "availability", "%15s", state);
 	if (err != 1) {
 		if (!xen_initial_domain())
 			printk(KERN_ERR "XENBUS: Unable to read cpu state\n");